C.a.R. > Applications > Conic Sections > Tangents to a parabola
This is a parabola as the track of all objects with equal distance to F and the line AB. The two tangents to the parabola belonging to A and B intersect at a point, which projects to the midpoint of the line segment AB. The proof is hidden. Right click to the construction and enable the display of all hidden objects.
To hide like this is a simple trick to let the user think before he sees the full construction. Another idea is to use a separate web page. Of course, unnecessary details (like the construction of the track) should be completely hidden.